1. Field of the Invention
The present invention relates to a phosphoric diester which has a UV (ultraviolet) rays absorbing group and which is useful as an active component of anti-suntan or anti-sunburn cosmetic compositions, a method of preparing the phosphoric diester, and cosmetic compositions containing the phosphoric diester.
2. Description of the Background
Recently, it has been found that ultraviolet rays are deeply involved in the mechanism of the onset of skin diseases such as skin cancer, suntan/sunburn, and aging of the skin. The increase in UV dose caused by the destruction of the ozone layer, which has now become a controversial issue, raises the problem of the possibility of harm by ultraviolet rays.
In view of the foregoing, protecting the skin from UV rays is desired, and accordingly, various cosmetic compositions such as anti-sunburn compositions, skin-care compositions, foundations and similar compositions which contain UV shielding substances have been put on the market.
Ultraviolet rays are divided into three groups: long-wavelength UV rays (UV-A; 320 to 400 nm), medium-wavelength UV rays (UV-B; 290 to 320 nm) and short-wavelength ultraviolet rays (UV-C; up to 290 nm). Among these, UV-A and UV-B both of which reach the surface of the earth have been found to be harmful to the skin. In order to protect the skin from these harmful UV rays, many UV shielding substances have been developed.
For example, inorganic compounds such as titanium oxide, iron oxide and zinc oxide, and oil-soluble organic compounds such as 4-t-butyl-4'-methoxydibenzoylmethane are known as UV-A shielding substances. Substances known to provide shielding from UV-B include p-aminobenzoic acid, salicylic acid, methoxycinnamic acid, and benzophenone derivatives. They are processed in various manners for enhancing the UV-shielding property or for providing a suitable material useful as cosmetic compositions. For example, the former are made into fine particles or ultrafine particles, subjected to hydrophobic surface treatment, or combined with other substances, and the latter are made into derivatives such as metal salts.
In processing to enhance UV-absorbing effects, safety to the skin and retention on the skin for long periods are especially important.
With regard, to safety, consideration must be given not only to the skin-irritating effect of the UV absorbing agent itself but also to irritation of the skin caused when the UV absorbing agent absorbs light. In order to solve this problem, an attempt has been made to avoid direct contact of the UV absorbing agent with the skin while the UV shielding effect of the agent is retained. For example, Japanese Patent Application Laid-open (Kokai) Nos. SHO 62-181213 (1987) and SHO 60-81124 (1985) disclose a method of intercalating UV absorbing agents into clay minerals such as montmorillonite. According to this: process, the safety of the agent to the skin can be enhanced by inserting UV absorbing agents into interlayers of clay minerals. However, this process is accompanied by the drawback that the intercalated UV absorbing agent is exchanged with molecules of oils or surfactants if a UV absorbing agent is blended with oils or is incorporated in an emulsion system, causing exuding of the UV absorbing agent and allowing contact of the agent with the skin.
Concerning the retention of UV absorbing agents on the skin, especially organic types involve the problem that their UV-absorbing effect does not last long because they are easily removed by perspiration or by secretion of sebum. In order to solve this problem, Japanese Patent Application Laid-open (Kokai) No. SHO 60-130655 (1985) discloses a technique in which powder particles are coated with UV absorbing agents which have been made into the form of water-insoluble metal salts. In this case, however, the UV shielding effect of the particles coated with UV absorbing agents is not necessarily satisfactory. When a better effect is desired, the quantity of powder particles must be increased, but this in turn leads to deterioration of feel upon use by giving a frictional sensation or lack of proper moistness.
Japanese Patent Application Laid-open (Kokai) No. HEI 2-251240 (1990) discloses a technique in which UV absorbing agents are encapsulated in microcapsules by the use of suitable microcapsulating agents. This involves the drawbacks that highly safe substances admitted for use with cosmetic compositions must be used as a microcapsulating agent, and that thickness and shape of microcapsules must be controlled so as not to decrease the UV shielding effect, which is technically difficult to meet.
Accordingly, UV absorbing agents which are very safe to the skin, which are retained on the skin for long periods and are capable of providing a long-term UV absorbing effect are still desired.
Under the above circumstances, the present inventors carried out extensive studies in an attempt to obtain a UV absorbing agent which is safe and has lasting effects, and as a result, have succeeded in introducing a UV absorbing group into phosphoric diesters known as very safe, and also have found that the thus processed phosphoric diesters form, together with various oils and other organic oil-based UV absorbers, an excellent thixotropic gel which is very useful as a UV absorbing agent when used as an ingredient of cosmetic compositions due to its safety to the skin, retainability on the skin and lasting ability of holding the oil-based UV absorbers. The present invention was accomplished based on these findings.